Macromolecules 1988,21, 1839-1846
1839
Macromolecular Stereochemistry: Effect of Pendant Group Structure on the Conformational Properties of Polyisocyanides1y2 Mark M. Green* and Richard A. Gross Department of Chemistry and Polymer Research Institute, Polytechnic University, Brooklyn, New York 11201 Frederic C. Schilling AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Karl Zero and Charles Crosby, 1113 Corporate Research and Development, Allied-Signal Corporation, Morristown, New Jersey 07960. Received August 2 1 , 1987 ABSTRACT The hydroxyl function of the S enantiomer of the 1,2-acetoneketal of propylene glycol was 6 (CHCl,)),which was polymerized by using NiCl, converted, via the amine, to the isocyanide ( [ a ] 5 4+32.9' isocyanide) ( [a]546 hexahydrate in the neat state to the derived poly((S)-2,2-dimethyl-l,3-dioxolane-4-methy1 +68.5O (p-dioxane);M, = 165000). The ultraviolet spectrum of this brown solid in CHC13exhibited a band, = 295 nm and 3.2 X lo2 L mol-' cm-', and a long as a shoulder on lower wavelength absorptions, with ,A, low-intensity visible region tail. The circular dichroism (CD) spectrum was similar to published spectra for a-substituted asymmetric polyisocyanides and showed three maxima (A,-, nm; Ae, L mol-' cm-I): 328, -0.082; 290, +0.015; 275,-0.047. High-field NMR studies exhibited extensive chemical shift dispersion for both the backbone carbon and the pendant carbons. Similar shift dispersion was encountered in poly(n-butyl isocyanide) and in poly(benzy1 isocyanide). Poly((R)-a-phenylethylisocyanide) also exhibited chemical shift dispersion superimposed on symmetrical but broadened lines in its '% NMR spectrum. Zimm plots constructed from wide-angle 633-nm light scattering data on poly((R)-a-phenylethylisocyanide) (M, = 107000) and on isocyanide) (M, = 165000) showed no angular dependence. The poly((S)-2,2-dimethyl-1,3-dioxolane-4-methyl weight average hydrodynamic radii of these polymers in tetrahydrofuran were determined by photon correlation respectively. If one adopts a 411 helix with a pitch of 4 %. proposed in the spectroscopy to be 51 and 24 %., literature for poly(a-phenylethylisocyanide),our data yield a persistence length of 32 A, which is substantially isocyanide). The results show that some larger than that for poly((S)-2,2-dimethyl-l,3-dioxolane-4-methy1 form of stereoirregularity,which could involve syn-anti isomerism about the carbon-nitrogen double bond, is seen in each of the polymers studied. In addition, the chain dimension is strongly dependent on the pendant group structure.
Introduction Millich and co-workers in Missouri: followed by independent efforts in demonstrated that strong acid promotes the polymerization of certain alkyl and arylalkyl isocyanides. Somewhat later Drenth and co-workers in Holland developed an additional route for the polymerization of isocyanides using Ni" salts.7 These polymers prepared by either route were assigned the poly-Schiff base structure on the basis of infrared bands analogous to models and to the known mode of reactivity of the functional group."' Thus the later literature, based on the assumption that this structurql characteristic would persist in all cases, designated these materials as poly(imin0methylenes).8 Millich's early concemQthat the unsaturation could shift to the pendant group led him to study the polymerization of the optically active a-phenylethyl isocyanide. The resulting polymer 1 exhibited a large increase in optical rotation over the monomer, which served to discount the concern and also suggested a regular conformation for 1.l0
N
II
+Ct,
1
Structural regularity in 1 was shown by space-filling models to be consistent with a 411 helix. Although this structure was supported by X-ray studies in the solid,
unoriented samples were examined, which necessitated assumptions about packing as a prerequisite to the helix assignment^.^*"-^^ For 1 prepared from the racemic monomer, the proposal of a 411 helix with a pitch of 4 A is consistent with the absolute intensity of X-ray scattering of toluene solutions. These solution data indicate a monomer projection length of 1.04 A,5J5J6 a value near to that expected for a helix with a pitch of 4 A. A regular helical conformation for 1 in solution would suggest a stiff polymer, and this characteristic is supported by solution properties of narrow molecular weight fractions.15J7 The Mark-Houwink exponent for 1 from the racemic monomer is 1.3 in toluene, and the second virial coefficient is nearly invariant in toluene over the Mnrange from 20 000 to 123000. In contrast, for the same samples the radii of gyration obtained from the angular dependence of X-ray scattering in toluene solutions were not proportional to the molecular weight13J5J6 over the range 20 000-90 000 (ATw), indicating some flexibility in the polymer chains. In a remarkable experiment, poly(tert-butyl isocyanide) of a number average degree of polymerization of 20 was resolved into optically active fractions.*8 From an analysis of the resulting circular dichroism spectrum the a u t h o r ~ l ~ , ~ assigned the helical sense and concluded that the earlier proposed 4/1 he1i~'l-l~ was applicable also to poly(tertbutyl isocyanide). Other data on polyisocyanides suggest that the structure of the pendant groups has a large effect on backbone conformation. The Mark-Houwink exponents for the polymers of 0-phenylethyl isocyanide (tetrahydrofuran)21 and racemic 2-octyl isocyanide ( t ~ l u e n e )are ~ ~0.68 , ~ ~and
0024-9297/88/2221-1839$01.50/00 1988 American Chemical Society
1840 Green et al.
1.75, respectively, both measured over approximately the same molecular weight range. These values, known to be sensitively related to molecular dimension^,^^ taken with the value of 1.3 for poly(a-phenylethyl isocyanide) (see a b o ~ e ) , ' ~suggest J~ that pendant group structure strongly affects chain dimensions. In recent years a large effort in synthetic chemistry has produced a wide range of polyisocyanides for general studies of the relationship of structure to optical activity and circular dichroism characteristic^^^,^^,^^ and as models for catalysis and various biological functions.26 Most of these systems, of quite variable structure, have been substituted on the carbon adjacent to the nitrogen (a-substitution) giving them a formal analogy to poly(aphenylethyl isocyanide) and poly(2-octyl isocyanide). The Mark-Houwink exponent of 1.75, for the poly(2-octyl isocyanide),22has been generally used to yield A& from viscosity data for these polymers, and the structures have been assigned as rigid 4/1 helical rods with a pitch of 4 A. In more recent efforts in this field a variety of polyisocyanides without a-substituents have been assigned a 4/1 helical rodlike c o n f o r m a t i ~ n . ~ ~ Circular dichroism measurements are used to support the assumption that a wide variety of optically active polyisocyanides with asymmetric pendant groups adopt a rigid 411 h e l i ~ . ~ ~Although , ~ ~ l ~ it has ~ * been ~ ~ suggested that an exciton is associated with the imino group chromophore near 300 nm and has been used to interpret the screw sense and helical preference of the 4/1 mode1,22y23,25,26 it is well-known that an exciton may arise from a variety of chiral conformations that may not involve extended helical forms.2s The assignment of a polyimine structure to the isocyanide polymers makes possible a unique polymer stereoisomerism associated with the carbon-nitrogen double bond. This question of syn-anti isomerism was addressed early in the literature but dismissed on the basis of space-filling models, although it was suggested that chain-breaking impurities could allow such isomerism.1° It seems important to increase our understanding of when this structural isomerism may be reasonably excluded. So far the only polyisocyanide studied extensively in solution is poly(a-phenylethyl isocyanide) initiated by strong acid.'@'' The possible stereoselectivity of this polymerization could greatly affect the dimensions of the polymer in solution in a manner similar to that observed on polyisocyanates.2 The change to Nin salt initiators and the use of enantiomerically pure monomer followed by studies of chain dimensions could therefore play a role in increasing our understanding of the conformational properties of these polymers. In this work we have prepared, using Nin initiation, three polyisocyanides without a-substituents, one of which is optically active. We have also prepared poly(a-phenylethyl isocyanide) from both racemic and optically active monomer. We hope our studies on these materials, reported below, will help to strengthen the basis for the structural characterization of this interesting class of polymers.
Results Synthesis. The optically active and racemic 2,2-dimethyl-1,3-dioxolane-4-methyl isocyanides were intermediates in the preparation of the analogous isocyanates and are described in detail in that work.2 These stereoisomers were polymerized by following the Dutch procedures7with NiC12.6H20in the neat state to yield, after purification by two precipitations, brown solids in moderate yield.29 These materials, (S)-2and (RS)-2were characterized by intrinsic second viscosity 7,weight average molecular weight Mw,
Macromolecules, Vol. 21, No. 6, 1988 Table I Characteristics of Monomer and the Polyisocyanides ( ( 5 ) - 2 and ( R 5 ) - 2 )
configuration at
monomer
C* Sd S' RSd
deg +35.8 +35.8
polymer
[425546,0 [(Y]*~~~; deg +68.5
Mwb
1O4AZb V , dL g-l 6.14 1.08'
165000
+68.0
0.08 0.61'
3.15
110000
In p-dioxane. In tetrahydrofuran by low-angle laser-light scattering. Units for A, are in m cm3 g-2. CIn CHC1, a t 25 O C ; intrinsic viscosities. Polymerized neat. e Polymerized in methanol.
4
m . 3
-
\ N
-2
3"
:
111 260
,
,
280
,
,
300
, ,
,
320
,
,
,
360
340
, ,
380
,
0
nm
Figure 1. Circular dichroism and ultraviolet spectra of (S)-2.
virial coefficient A Z ,and optical activity for (S)-2,and these data appear in Table I.
N
214 ,
2PSl
Ultraviolet and Circular Dichroism Spectra. Circular dichroism (CD) has been utilized to determine the helical p r e f e r e n ~ e ~in ,optically ~ ~ ~ ~ active ~ ~ ~a-substituted ,~~ polyisocyanides, suggesting a comparable measurement for (5')-2. The CD and the associated ultraviolet (UV) spectra are exhibited in Figure 1. The circular dichroism literature is based on an analysis of the spectrum of the resolved
Macromolecules, Val. 21, No. 6, 1988
Conformational Properties of Polyisocyanides 1841 THF
I'
THF
CARBON ' a '
f' f,f
